The disclosure relates to the field of electronic reproduction technology and relates to a method for calibrating an engraving amplifier in an electronic engraving machine for engraving printing cylinders for gravure printing.
In an electronic engraving machine, an engraving member with an engraving stylus as a cutting tool moves at a rotating printing cylinder in the axial direction. The engraving stylus, which is controlled by an engraving signal, cuts a series of cells into the surface of the printing cylinder. The engraving signal is formed in an engraving amplifier by superimposing image signal values with a periodic vibration signal. While the vibration signal effects an oscillating lifting motion of the engraving stylus for the purpose of generating the engraving raster, the image values which represent the tone values between “light” and “dark” which are to be reproduced determine the geometric dimensions of the engraved cells.
So that the cells that are engraved on the printing cylinder have the desired tone values as prescribed by the image signal values, a calibration of the engraving amplifier is performed. For this purpose, in a test engraving process test cells are engraved for prescribed desired tone values, for instance for the desired tone values “light”, “dark” and “middle tone”. After the test engraving, the geometric actual dimensions of the engraved test cells are measured out and compared to the corresponding desired dimensions. Settings are calculated from the comparison of the geometric dimensions, with which settings the engraving signal is calibrated such that the geometric dimensions of the cells that are actually created in the engraving correspond to the geometric dimensions required for an engraving with the correct tone values.
In conventional calibration of an engraving amplifier of an engraving machine, the settings are prescribed, the geometric dimensions of the test cells that are engraved in test engravings are measured out, and the new settings are calculated with the aid of the measurement results essentially manually, with the setting processes and subsequent test engravings being continued until an optimal calibration is reached. A disadvantage of the conventional procedure is that the operator must have practical experience concerning the relationships between the electrical setting points and the actual geometric dimensions, which are to be expected, of the test cells, whereby numerous parameters and marginal conditions must be accounted for, such as the transmission behavior of the engraving amplifier and the engraving member, the cut angle and the degree of wear of the engraving stylus, as well as the material hardness of the surface of the printing cylinder that is to be engraved.
EP 0 595 324 A already reaches a method for calibrating an engraving amplifier of an engraving machine in which signal values for modifying at least one parameter “vibration”, “light”, “dark”, or “medium gradation” are set at the engraving amplifier, a test engraving is performed using the adjusted signal values, the actual dimensions of the engraved test cells are measured out, and, from the measured actual dimensions and the predetermined desired dimensions, difference values with which the signal values are corrected are calculated upon consideration of previously calculated transmission functions, with the individual steps being repeated in a routine using the respectively corrected signal values until the actual dimensions of the engraved test cells are within a tolerance range.
A similar method for calibrating an engraving amplifier of an engraving machine is known from U.S. Pat. No. 5,438,422 A.
The known methods have the disadvantage that optimal calibration requires a relatively long time, since a new test engraving must always be conducted in the repetitions of the steps.